The present invention relates to process for producing water-absorbent polymers of improved quality which have broad utility not only for sanitary materials such as physiological napkins, paper diapers, and the like, but also in a variety of fields such as farming and gardening, foods, and the like.
In recent years, water-absorbent polymers have been used not only for sanitary materials such as physiological napkins, paper diapers, and the like, but also for materials, which require water-absorbent and water-holding properties, such as water-holding agents for use in farming and gardening.
As the water-absorbent polymers of that kind, for example, there have been known the cross-linked products of partially-neutralized polyacrylic acid, hydrolysis products of graft polymers of starch with acrylonitrile, neutralization products of graft polymers of starch with acrylic acid, saponification products of copolymers of vinyl acetate with acrylic acid esters, and hydrolysis products of copolymers of acrylonitrile or acrylamide or their cross-linked materials.
However, it has also been known that these water-absorbent polymers have a problem that, due to the known blocking phenomenon among the polymer particles under a high humidity condition, the fluidity of these particles becomes lower and handling becomes difficult. Although several attempts to improve the fluidity of these particles of water-absorbent polymers have been carried out (Japanese official patent provisional publications, showa 56-103208 and 61-97333, and U.S. Pat. No. 4,295,987), satisfactory improvement has not yet been achieved because of complications of handling and decrease of the water-absorbent capacity.
On the other hand, in the Japanese official patent provisional publication, showa 62-7745, there has been disclosed a method wherein, to prevent the blocking of particles of water-absorbent polymers, a quality-improved water-absorbent polymer is produced by spraying an aqueous solution of aluminum sulfate on a water-absorbent polymer, using an apparatus for spraying and drying. Also, in the U.S. Pat. No. 4,043,952 there has been disclosed a method wherein a quality-improved water-absorbent polymer is produced by surface treatment of the polymer particles in a dispersion medium containing a polyvalent metallic ion, such as aluminum etc. (for example, a mixed solution of methanol and water). However, there are limitations to these methods because a complex apparatus, such as an apparatus for spraying and drying, must be used, or a specialized dispersion medium must be used and, therefore, particles of a quality-improved water-absorbent polymer could not easily be obtained.
Also, in the Europian patent provisional publication 0268459A2, there has been disclosed a method wherein, to prevent deterioration and decomposition, with the passage of time for a swelling gel of a water-absorbent polymer, a quality-improved water-absorbent polymer is produced by spraying and mixing an aqueous solution of a reductive agent containing sulfur (for example, sodium thiosulfate etc.) with the water-absorbent polymer particles. However, this method does not improve the fluidity and the property for resisting blocking of a water-absorbent polymer itself.
Besides, the existence of residual unpolymerized monomers in these water-absorbent polymers, especially when such polymers are used as sanitary materials or in the field of foods, are drawing attention on a side of safety, so that several attempts to decrease the content of residual monomers have been carried out. For example, there have been proposed a method which lowers the residual monomers by adding an amino acid to a water-containing gel of a water-absorbent polymer (U.S. Pat. No. 4,766,173) and a method which lowers the residual monomers by illuminating a water-containing gel with an ultraviolet light (Japanese official patent provisional publication, showa 63-260907). However, these methods have not showed a sufficiently lowering effect on the residual monomers and, if an attempt is made to decrease the residual monomers to a standard of enough safety, a lot of investment and work is required leading to an unpractical application.